Telephone subset microprocessors require relative high current for operation and normally, when one such subset is connected to a single exchange line and is in the on-hook mode, the microprocessor is energized by energy stored in a storage capacitor. The voltage across the storage capacitor is monitored such that when it is below a predetermined value a control circuit operates the line switch to rapidly charge the storage capacitor to a voltage sufficient to maintain the microprocessor. Thereafter the voltage on the storage capacitor is maintained via a bleed circuit.
When, however, two or more such subsets are connected in parallel to a single exchange line, a problem arises in providing sufficient energizing power to the microprocessor of any subset in the on-hook mode when one other subset is in the off-hook mode. Momentarily operating the line switch, as described above, in the subset/s in the on-hook mode is highly undesirable because of interference which this would cause to the transmission from the off-hook subset.